1. Field of the Invention
The present invention relates to a radar system mounted on a vehicle, and more particularly to a radar system mounted on a vehicle such as an automobile and used for measurement of a distance between vehicles.
2. Description of the Related Art
As such a general radar system mounted on the vehicle, there have been known many radar systems which are downsized by using a transmit/receive shared antenna and improves in the mounting property of a subject automobile. In the radar system of this type, an electromagnetic wave of a frequency of, for example, a transmitting frequency ftx=76.5 GHz is outputted from an oscillator, and the electromagnetic wave is allowed to pass through a power divider and amplified by a transmitting antenna. The electromagnetic wave amplified by the transmitting amplifier is allowed to pass through a transmit/receive changeover switch and is outputted to a space from the transmit/receive shared antenna. Also, the electromagnetic wave outputted to the space from the transmit/receive shared antenna is reflected from a target object that exists at a distance R, and then, input to the transmit/receive shared antenna with a delay time Δt that depends on the distance R with respect to the transmitting electromagnetic wave. Also, when the target object has a relative velocity, a receive electromagnetic frequency is Doppler-shifted by a given amount with respect to the transmit electromagnetic frequency, and then, input to the transmit/receive shared antenna. The relative distance and the relative velocity of the target object are calculated on the basis of the receiving electromagnetic wave input to the transmit/receive shared antenna.
As another conventional radar system mounted on a vehicle, there has been also known a car-to-car distance measuring device using a pulsed laser beam that detects interference due to a receive level of an incident light and periodicity of detection or a variation of the measured distance (for example, refer to Japanese Patent No. 03185547).
However, the above-mentioned conventional general radar system mounted on the vehicle has an S/N ratio deteriorated upon receiving an interference wave of another radio device or another automobile radar, resulting in a case in which detection may be in failure or a detection error may occur. Thus, there arises such a problem that a vehicle control system or an alarm system fails.
Also, in the radar system disclosed in Japanese Patent No. 03185547, a car-to-car distance measuring device obtains a distance at the transmit/receive time interval of a pulse light when receiving the pulse light that is equal to or higher than a predetermined threshold value with respect to one pulse transmit light. Therefore, in the case where the car-to-car distance measuring device receives an interference pulse before receiving a reflection light from the target after transmitting the pulse light, there arises such a problem that a receive light level is relatively constant and large and a distance value is inaccurate. The interference is detected by the variation of the receive level or the distance by using that characteristics. However, because the radar using an electric wave and the radar using the light are different from each other in a principle for detecting the reflection wave, Japanese Patent No. 03185547 arises such a problem that the electric wave type radar does not function accurately. In particular, in the case of the electric wave, because the interface wave and the transmit wave are subjected to various modulations such as frequency modulation, pulse modulation or phase modulation, an output signal of the interference wave is different depending on a combination of those modulations. Also, in the case of the radar using the electric wave, there arises such a problem that the interface wave input within an observation time reliably induces deterioration of detection performance or detection error.